1. Field of the Invention
This invention relates to a semiconductor device capable of controlling electrically a light absorption coefficient using a quantum well and, more particularly to a light absorption control semiconductor device for changing characteristics such as frequency modulation, intensity modulation, switching, and filtering a propagating light by controlling electrically the light absorption coefficient using the quantum well.
2. Description of the Related Art
There has been conventionally known a semiconductor device including quantum wells enclosed by energy barriers of junctions of different semiconductor materials (heterojunctions). By narrowing the width of this quantum well, quantized energy levels are formed at the quantum wells. There have been proposed a variety of devices taking advantage of the discreteness of the quantized energy levels. A resonant tunneling diode, a resonant tunneling transistor, and the like are known as examples of these devices. These devices take advantage of the fact that the quantized energy level is changed by applying an electric field in a direction perpendicular to the junction to thereby produce a resonant state which brings about a tunnel effect between two layers holding each barrier therebetween.
Further, concerning the light absorption characteristic, the light absorption under the influence of direct transition is being studied between the quantized energy level in a valence band and the quantized energy level in a conduction band using two quantum wells formed of the same material and having the same width (Appl. Phys. Lett. 50(16), 20 Apr. 1987, P1098). According to this reference, the energy of two quantum wells are the same in both the conduction and valence bands when the electric field is not applied thereto, thus realizing the resonance state where the wave functions of electrons interact.
However, in a state where the quantized energy levels at the two quantum wells match in the conduction and valence bands, these two quantum wells are equivalent to a single quantum well having width equal to a sum of the widths of the two quantum wells. The light absorption coefficient is not necessarily large in this state. The above reference analyzes the fact that light absorption is unlikely to occur since the electrons localize in one of the quantum wells when the electric field is applied to these two quantum wells, which are identically structured. However, this mechanism is not always clear.
On the other hand, there has been known a semiconductor device including a semiconductor layer having a function obtained by growing a compound semiconductor on a substrate by an epitaxy method, and then forming lower and upper electrode layers holding the semiconductor layer in parallel from opposite sides (in parallel with the substrate surface). For example, a light emitting diode, a laser diode, and a resonance tunnel diode are known as examples of these devices. In these semiconductor devices, the lower electrode layer is formed of a conductive compound semiconductor of a thickness of about 1 .mu.m in which impurities of a concentration of 1.times.10.sup.17 cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3 are included.
However, in these semiconductor devices, it is necessary to grown an epitaxial semiconductor layer having a device function on the lower electrode layer. This degrades the crystallinity of the conductive semiconductor layer in which the impurities are doped, and accordingly the important semiconductor layer having the device function formed on the doped layer by the epitaxy method exhibits poor crystallinity.
According to experiments conducted by the inventors of the present invention, in the case where a GaAs semiconductor layer has a device function on an n.sup.+ -GaAs layer having an impurity concentration of 3.times.10.sup.18 cm.sup.-3 and a thickness of about 1 .mu.m and a multiple quantum well, it was found that the crystallinity of the multiple quantum well was poor based on the measured half value width of a photoluminescence spectrum. Since the crystallinity of the GaAs semiconductor layer having the device function is poor, the device characteristics are also poor.
The spectrum of the photoluminescence intensity of the n.sup.+ -GaAs layer is as shown in FIG. 11. Specifically, there are obtained peaks at 805 nm (indicated at a), 820 nm (indicated at b), and 832 nm (indicated at c). The peak c is an absorption peak due to the transition of carriers between a donor level and an acceptor level; the peak b is an absorption peak due to the direct transition of carriers between the bottom of the conduction band and the top of the valence band; and the peak a is an absorption peak due to the transition of carriers between a higher position in the conduction band and the valence band resulting from an improvement in a Fermi level caused by the doping of the high concentration impurities. It is not desirable that the light absorption occurs in the band shorter than 800 nm when the light absorption control device is constructed using the multiple quantum well of the present invention to be described later.